Can you change the damage type of Poisons via Grave Touched?

Question

If I hit a creature with an attack using a poison coated weapon (for example Purple Worm Poison) could I, rules as written, change the poison damage type to necrotic via the lvl 6 Undead Patron Warlock's feature Grave Touched? That reads:

[...] once during each of your turns, when you hit a creature with an attack and roll damage against the creature, you can replace the damage type with necrotic damage. While you are using your Form of Dread, you can roll one additional damage die when determining the necrotic damage the target takes.

There seem to be two conditions for the feature to be able to convert damage:

1. The hit with an attack ("when you hit a creature with an attack")
2. The roll of damage against the creature that was hit. ("and roll damage against the creature")

But it doesn't state, that the damage roll must be part of the attack that fulfilled condition 1. For comparison the lvl 3 Horizon Walker feature Planar Warrior reads:

The next time you hit that creature on this turn with a weapon attack, all damage dealt by the attack becomes force damage [...]

Here the damage to be converted explicitly must have been dealt by the attack.

Thus, if the damage roll in Grave Touched doesn't have to be dealt by the attack, could you choose which roll of damage you would convert as long as it happens in the same turn as the attack fulfilling condition 1. ? Or would it have to be the first instance of a damage roll after the hit? If so, could you use the blowgun and poison coated needles, since they don't involve a "roll" of damage but only a flat 1 point of piercing damage, making the applied poison damage the first instance of a damage roll after the hit?

Answer 1

Grave Touched should replace all of the attack's damage with necrotic damage.

Before we look at the specifics of Grave Touched, I want to make an observation about a spell that also replaces damage. Consider the spell lightning arrow:

The target takes 4d8 lightning damage on a hit, or half as much damage on a miss, instead of the weapon’s normal damage.

With lightning arrow, the description takes care to state that only the weapon's normal damage is replaced - using lightning arrow with a piece of poisoned ammunition would deal 4d8 lightning plus the poison damage.

Now, observe Grave Touched:

when you hit a creature with an attack and roll damage against the creature, you can replace the damage type with necrotic damage.

Grave Touched tells us to replace "the damage type", making no consideration for an attack dealing multiple damage types, nor specifying that it only replaces the "normal" damage type. From this, we conclude Grave Touched does not care about the damage type composition of the attack, it just all becomes necrotic damage.

However, Purple Worm Poison is not a part of the attack's damage.

Purple Worm Poison states:

A creature subjected to this poison must make a DC 19 Constitution saving throw, taking 42 (12d6) poison damage on a failed save, or half as much damage on a successful one.

The damage from the poison is not a part of the damage roll for the attack - first the attack must hit, and the attack must deal damage, then the target must make a saving throw to be subjected to the poison's effects. The damage from the attack is one of the conditions for triggering the poison, so the attack's damage must be entirely separate from the damage from the poison. So Grave Touched would not change the damage type of the poison damage from the weapon poison.